Application Examples

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How to measure the friction and adhesion of skin creams

WHY ?  : Skin creams are commonly used to improve skin health and create a smooth, soft, and moist perception. This is achieved by altering the surface roughness, friction, and adhesion of skin surface. Despite the fact that there are many commercial creams available, there is no consistent approach to determine their frictional and adhesive properties.

HOW ?  : Two different test procedures are developed to evaluate the friction and adhesion of commercial skin creams. To investigate the frictional behavior, a reciprocating sliding was performed by applying a film of cream on an artificial skin. As a counter material, silicone was used to simulate the actual tribosystem. Different sliding speeds, applied loads and sliding distances can be used to evaluate and compare different contact conditions. In addition, the adhesion of the skin creams was measured based on approach-retraction curves. In this procedure a silicone counter body gradually approaches the artificial skin with the cream, until a pre-set contact load is reached. Then, the silicone moves away from the greased substrate under well controlled conditions, until complete physical separation is achieved.  During this approach-retraction cycle, the force on the load sensor is measured as a function of time and distance moved. 

 

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RESULT  :

- The frictional behavior of various creams can be investigated in an accurate and efficient way, under realistic conditions.

- The same apparatus and setup can also be used to measure the adhesion and separation energy of creams on artificial skin. 

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Friction and wear of thin layers for MEMS

WHY ?  : Evaluating frictional and wear characteristics of very thin nanostructured layers with macro scale tribometers, in the Newton load range, can create unrealistic conditions.  Wear phenomena are highly dependent on the contact conditions: such high loads are not relevant in the case of MEMS. The adhesive and capillary components that contribute to friction, in a micro-contact, can not be simulated with high load devices.  Therefore, there is an increasing need to use new tribological testers and procedures to obtain a better understanding of surface interactions on an appropriate scale.

HOW ? : The Basalt-N2 tribometer can bridge the gap between the macro-load (conventional pin-on-disk) and nano-load (atomic force microscopes AFM) tribometers. Its versatile loading system, and by selecting cantilevers or strain gauges a load range of 0.2 mN up to 100 N is possible. In the case, loads between 500 mN and 2 N were investigated. Different contact geometries (point, line, area contacts) and sliding velocities can also be used. Due to the high sensitivity of this tester the transition between different phases can be successfully recorded (e.g. sliding between coated and uncoated components).  

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RESULT :

- Conventional macro-load scale wear testers are not suitable for studying the wear behavior of thin layers, because the high initial contact pressure results in severe deformation and/or fracturing of the coating.

- Meso-load testing was useful as it allowed to record accurately the frictional behavior of the coating without damaging it, and with a minimum substrate effect.

- Thanks to the high sensitivity of this meso-load tester, surface phenomena such as oxidation and/or debris formation can be easily detected by monitoring the evolution of the coefficient of friction of the tribosystem. 

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High temperature sliding wear testing of materials

WHY ?  : High temperature tribological testing often requires the development of complex mechanical setups, that should meet rigorous standards and specific performance metrics. Thus, the development of a state-of-the-art experimental setup to study the reciprocating sliding behaviour of various bulk and coated materials at temperatures that can reach up to 1000 °C is needed, especially for the evaluation of high temperature materials for aeronautical applications.

HOW ?  : A macro-load tribological tester was developed at the Materials Engineering Department (MTM) of KULeuven, which allows for the evaluation of the reciprocating sliding behavior of bulk or coated materials at temperatures up to 1000 °C. In particular two different series of heating profiles can be implemented, namely under elevating or isothermal temperatures. The selection of the first methodology was based on the hypothesis that the coefficient of friction of a tribosystem strongly depended on the bulk and surface characteristics of the materials. Indeed, the formation of e.g. an oxide film at the interface of the contacting materials will result in a change of the coefficient of friction, and thus a first indication of the temperature at which this phenomenon took place, will be drawn. Therefore by performing elevating wear tests it is easier to pin point the temperatures at which the progressing phenomena affect the tribological behaviour of the tribosystem and define the temperature limits at which a material can be functional. On the other hand, isothermal tests are used to investigate theinfluence of structural changes and oxide formation of thermally stable systems and provide additional information to the elevating temperature tests. Combination of these profiles is also possible (e.g. cyclic heating). 

 

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RESULT  :

- A methodology was established to evaluate the effect of temperature on the friction and wear of industrial coated or bulk materials. Different heating profiles can applied to simulate the actual environment of the application.

- Elevating temperature profiles can be used to pin point the temperatures at which the progressing phenomena (e.g. phase transformation, oxidation) affect the tribological behaviour of a tribosystem and define the temperature limits at which a material can be functional.

- Isothermal heating profiles are used to evaluate the tribological behavior of thermally stable systems. 

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Thin layer activation (TLA) technology for on-line wear measurements

WHY ?  : The reliability of industrial equipment, transportation systems, nuclear and conventional power plants etc. can be significantly influenced by surface phenomena such as corrosion and wear. With the increasing pressure on development time and the need for higher performance, there is also an increasing need to measure and quantify the degradation phenomena faster and better. In this perspective, nuclear activation technology - as already used in engine testing- can provide accurate in-situ measurement and precise monitoring of wear, mass transfer, corrosion and erosion.

HOW ?  : It is important to investigate the wear behavior of tribosystems under moderate or low load conditions, where the wear rates can be extremely low and cannot be easily measured with conventional techniques. As a case study TLA technique was incorporated to a Falex Block-on-Ring to simulate wear damage of a journal bearing in a crankshaft under lubricated conditions.

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RESULT :

- TLA can be easily implemented in a lab environment .

- Accurate in-situ measurements of extremely low wear rates can be achieved.

- The run-in and steady state wear evolution can be observed.

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Friction modifiers put to the test. Can we influence friction?

WHY ?  : In the effort to reduce CO2 exhaust, an important approach is to reduce friction in the engine.  One part of the mix of options are ‘friction modifying additives’, such as the well-known GMO, which are known to reduce friction by 5, 10 or 20%. However, the difficult task is to prove the effect of friction modifiers in the engine, since existing engine tests measure the interaction of all sliding and moving components, as well as lubricant viscosity and other effects. In order to isolate and evaluate the efficiency of friction modifiers, a precision frictional approach is required. 

HOW ?  : The high precision tribometer Basalt-S2 was used. Applied loads and friction are measured with mN precision, using a ball-on-flat contact geometry. This creates realistic contact pressures.  Due to the high sensitivity of this tester, differences between the base oils and friction modifiers were successfully recorded.  

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RESULT :

- The effect of different modifiers can be separated by the precision microtribometer.

- Measurements are repeatable enough to draw significant conclusions.

- A ranking of base oils and oils containing friction modifiers is reached.

- A friction reduction of 10 to 18% in the moving contact is possible with the use of the right friction modifier.

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Thermosetting polymers for high speed bearings : linking friction and heat

WHY ?  : Polymeric materials are used more and more as cage material for light weight bearing applications, but thermoplastic materials suffer from PV limits.  At high speeds, the polymer may melt easily under light loads.  Thermoset resins don't have this limit, but may still disintegrate under higher temperatures.  In this method, we can apply high speeds and variable loads, to explore the limits of thermosets.

HOW ? : The Falex Block-on-Ring tester has a simplified journal bearing geometry and consists of a stationary block loaded against a rotating or oscillating ring. A stable line contact is created, where speed, load, temperature, angle of oscillation, specimen material and finish can be varied. For this work, high speeds are of interest. Friction forces are low, but the accumulation of temperature is giving useful information about the amount of friction dissipated in the contact, and whether the materials can withstand these conditions.  Comparative testing can show the advantages of thermosets over thermoplastics for high speed applications.

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RESULT :

- Ranking of different polymeric materials under experimental conditions that simulate the actual application is possible.

- The evolution of friction can be linked to the temperature, applied load and sliding velocity.

 

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How can we measure the stickiness of clay ?

WHY ?  : During the processing of bricks in the construction industry, clays slurries can adhere (stick) to mechanical components such as mixers, hindering their function. In addition, in the drilling industry severe damage of the drills can be caused by the sticking and swelling (due to water adsorption) of soils onto the drills. A methodology needs to be developed to measure the stickiness of clays/soils on metallic components.     

HOW ?  : A test procedure is established, based on approach-retraction curves, by using an upgraded TETRA Basalt-N2 micro-tribometer, with a Millinewton light load sensor. In this procedure a user selectable indenter body (ball, pin) gradually approaches the clay slurry/soil layer until they come into contact, then the indenter body keeps moving down until a pre-set contact load is reached. Then, the indenter body moves away from the greased substrate under well controlled conditions, until complete physical separation. During this approach-retraction cycle, the force on the load sensor is measured as a function of time and distance moved. This technique is the same as pull-off force experiments with an atomic force microscope for studying physical interactions.

 

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RESULT :

- The stickiness of clays can be accurately measured.

- Repeatable measurements were obtained with the Basalt-N2.

- The effect of water content on the stickiness of the clays can be measured.

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Tribological behavior of lubricating tribo-systems for the watch industry

WHY ?  : One of the main issues in the watch industry is reduce the friction and sticking between moving components. To achieve this, a small quantity of lubricant is added in the contact. However, due to the high expectations of the costumers, the increased lifetime of the watch, the size and geometry of components and contact conditions (loads in the mN range), there is a huge need to develop a tool that can evaluate such lubricating tribo-systems. The main challenge is to perform precision frictional measurements, in conditions that simulate the “actual” application.

HOW ?  : The updated Basalt-N2 tribometer can perform precision frictional measurements in the load range of 0.2 mN up to 1N, which is relevant for such applications. Different contact geometries (point, line, area contacts) and sliding velocities can also be used to simulate different contacting components within the watch. Due to the high sensitivity of this tester the tribological phenomena such as sticking can be picked up. There is also a dwell (stop) time option, to simulate periodical movements e.g. hour and/or date change mechanisms, whereas the actual components can also be fixed onto specially design holders (customizing according to end-user).            

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RESULT :

- The friction of lubricating tribological contacts in the mN range can be measured with precision.

- Repeatable measurements were obtained with the Basalt-N2.

- The testing parameters can be fined tuned to simulate the actual component conditions.

- The effect of loading, sliding speed and dwell time on the friction of tribological contacts for the watch industry can be measured.

- Comparison between different lubricants, materials and coatings is possible due to the precision of the sensors.

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Composite polymers: How do they perform in water “tribocorrosion”

WHY ?  : Polymer based composites are considered as one of the most important engineering materials for naval applications. They can be used in the superstructures, decks, bulkheads, advanced mast systems, propellers, propulsion shafts, rudders, pipes, pumps, valves, machinery and other equipment on large ships. In the majority of these applications these composites are subjected to mechanical loading in a corrosive environment. Thus their performance and/or lifetime is strongly dependent on both of these factors. In this application a methodology was developed to evaluate the effect of the corrosive environment (seawater) on the tribological performance of composite polymers is sliding contacts.  

HOW ?  : A Basalt-S2 was used to measure the friction between polymer composite blocks, having different compositions and a metallic countermaterial (toll steel bearing). The aim is to evaluate how the degradation (corrosion and wear) of both the composite and the metal influences the tribo-systems properties. The influence of chloride ions, which are known to accelerate corrosion, were investigated by comparing the friction of the tribosystem when sliding in distilled water and in seawater. A µSurf confocal microscope was used to measure the wear damage on both the ring and blocks.               

 

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RESULT :

- The friction of polymer composites in distilled and seawater were measured.

- The effect of the corrosive environment on friction can be recorded.

- Changes in the evolution of the friction can be linked to the formation of corrosive products on the contacting interface.

- Comparison between different materials, motions (unidirectional or reciprocating), speeds, and geometries is possible with the Basalt-S2.

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Cost efficient data collection for statistical analysis of wear - TRL 6

WHY ?  : One of the most difficult industrial issues related to tribology is the prediction of long term wear or material durability.  In many components and products, materials with or without lubrication are used to reduce wear and maintain functionality of the component.  Required ‘wear life’ may be thousands of hours.  Contrary to the determination of a ‘coefficient of friction’ – which can be done in a few hours, the determination of wear and wear rate under realistic conditions is a long term test. The challenge is twofold : perform low wear rate experiments with many repeats at an economically acceptable cost.  The only way to do this is by a multistation approach (performing many wear experiments simultaneously). 

HOW ?  : Parallel tests were performed in our TRL6 prototype 10-station cross-cylinder block-on-cylinder tester. With this method, we test parallel and simultaneously different bulk or coated materials (metals, alloys, polymers, ceramics and composites), at moderate contact pressures and for a prolonged period of time. Adhesive or mild abrasive wear mechanisms are representative for the “actual” applications.

  • Up to 9 kilometers of sliding distance can be realised in a single day, on 10 wear contacts simultaneously.
  • To measure the wear damage, we use weight loss measurements, optical and/or confocal microscopy.  10 data points collected efficiently

 

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RESULT :

  • The wear of various materials can be measured in a time efficient and economical way, realistic wear rates simulate actual applications.
  • Statistical analysis of the wear data provides a higher confidence level and allows outlier analysis.
  • Reliability testing of materials becomes economically possible.

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